The rate of hydrolysis of 2-naphthyl acetate catalyzed by R-chymotrypsin has been measured in aqueous solutions of dodecyltrimethylammonium bromide at concentrations below and above the critical micelle concentration, as well as in the absence of surfactant. Under all the conditions employed, the reaction takes place following a Michaelis-Menten mechanism. The presence of the surfactant, at concentrations above its critical micellar concentration, increases the value of the Michaelis constant, Km, without significant changes in the catalytic rate constant, kcat. The increase in Km is larger than that expected from the incorporation of the substrate to the micellar pseudophase, indicating that the interaction between the enzyme and micellelike aggregates alters the formation of the enzyme-substrate complexes. This can be related to a partial unfolding of the enzyme, as is suggested by changes in its intrinsic fluorescence.

The partition of 1-pyrenemethanol (PyM) between the external medium and AOT/water reverse micelles formed in several organic solvents has been measured and compared with its partition between bulk solvents. The organic solvents employed were n-heptane, cyclohexane, isooctane, dodecane, benzene, chlorobenzene, toluene, and tetrachloroethylene. The partition of PyM in AOT/water/nonpolar solvent reverse micelles is dominated by the free energy of the probe in the external solvent. To evaluate the stability of the probe in micelles dispersed in different solvents, the micellar data were referred to a common (water) solvent. A weakly positive correlation was obtained for the partition of the probe between the different micelles and water. Furthermore, the polarity sensed by PyM is higher in AOT/alkane/water than in AOT/aromatic solvent/water reverse micelles. These results can be explained in terms of two driving forces whose relative importance is determined by the characteristics of the solute and the external solvent. One of them, solvation of the surfactant head by the polar group of PyM, will be disfavored by polar solvents. The second one, interaction of the aromatic part of the micelle associated PyM molecules with the external solvent, would be determined by the solvent/probe affinity, measured by the solvent/ water partition constant.

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The rate of hydrolysis of p-nitrophenyl acetate (PNPA) catalyzed by Mucor javanicus lipase has been measured in AOT reverse micellar solutions formulated in aliphatic hydrocarbons, aromatic hydrocarbons and a chlorinated compound. The study has been performed at a single value of W=([water]/[AOT])=6.0. Fluorescence decay measurements of intrinsic enzyme fluorescence, mainly due to tryptophan residues, in the different reverse micellar systems were also carried out, in an attempt to obtain some insight on the effect of the organic solvent on the protein conformation. Differences observed in the kinetics of the fluorescence decays of tryptophan residues of the lipase incorporated to the micelles with the different external organic solvents were also found in the catalytic behaviour of the enzyme. In particular, it is observed that the contribution of the long lived component of the fluorescence decay is considerably higher (ca. 40%) in aliphatic than in aromatic solvents (ca. 15%), indicating significant differences in the protein conformation. This effect of the organic solvent on the protein conformation is also observed in the enzymatic activity, which is higher in the aromatic than in the aliphatic solvents.